The invention relates to a magnetic resonance apparatus which comprises an essentially cylindrical magnet system for generating a steady magnetic field in a measurement space, a coil system for generating gradient fields in the measurement space, and at least one RF coil which is arranged in the immediate vicinity of the measurement space, approximately annular first elements of a magnetically conductive material being provided coaxially with the axis of the magnet system in order to increase the homogeneity of the steady magnetic field within the magnet system.
1. Field of the Invention
An apparatus of this kind is known from EP-A-0 216 404 which corresponds to U.S. Pat. No. 4,771,243: In the known apparatus the first elements are formed by rings of a ferromagnetic material which are arranged in recesses in the coil former for the gradient coils. A comparatively large amount of ferromagnetic material is required for this purpose. The cited document specifies that the rings should have an axial dimension of between 1 and 10 cm and a thickness of up to 2 cm. Furthermore, adaptation of the magnetic field correction provided by the first elements to changing circumstances is virtually impossible. This becomes apparent, for example from the fact that the document states that a compromise must be found for magnet systems required to operate with different field strengths. The correction provided by the first elements, therefore, is not optimum for any of the field strengths used.